1. Field of the Invention
The present invention relates to a method for reducing the viscosity of kaolin clays containing mineral slime impurities, and for increasing the brightness of these clays. More particularly, the present invention relates to a method for removing mineral slimes from a kaolin clay.
2. Description of Related Art
In many applications, the value of a kaolin clay depends both on its viscosity in aqueous slurries and on its color or brightness. Natural occurring kaolin clays can vary considerably in their color properties, even when produced from mines in the same locality or even from different sites in the same deposit. Discoloring contaminants found in natural occurring kaolin deposits often include titanium minerals and organic (carbonaceous) impurities. A clay otherwise suitable for a specific application may be rejected solely for its color.
U.S. Pat. No. 3,371,988 describes a process for removing highly colored titanium impurities from a kaolin clay by overdosing with a peptizing agent. The patent teaches dispersing kaolin clay slurry with the peptizing agent, such as sodium hexametaphosphate, in an amount of about twice that required to produce the minimum clay viscosity prior to classifying the clay. The colored titanium impurities then are separated as a sediment from the refined kaolin supernatant slurry.
U.S. Pat. No. 3,661,515 is directed to a method of brightening organically contaminated kaolin clays by oxidation. This method involves the procedure of chemically dispersing the clay into an aqueous slurry, using for example sodium hexametaphosphate, followed by degritting. After degritting, the slurry is treated with an oxidizing agent such as sodium hypochlorite followed by settling for a period of time which permits oxidation of organic contaminants and deposition of organic particles with larger clay particles. The supernatant kaolin clay slurry then is separated from the sediment containing the organic contaminants and the recovered slurry is flocculated, washed, filtered and dried by conventional methods.
There also are many kaolin deposits which cannot be used as a source of clay for many applications because of the relatively high level of certain mineral constituents, such as smectites (montmorillonite), present in the clay. These mineral constituents appear as very small or hyperfine particles in a slurry of the clay, and will be referred to herein as mineral slimes. The presence of a high level of mineral slimes in a kaolin may cause the clay to have an excessive viscosity in aqueous slurry and often contributes to its having an undesirable degree of color. High levels of mineral slimes also contribute to increased consumption of reagents during beneficiation and afterwards when such clays are employed in specific applications.
While attempts have been made to upgrade such marginal and submarginal kaolin by removal of mineral slimes, these attempts have not proved sufficiently successful for widespread commercial application. Often, these processes have achieved only limited removal of slimes, have not achieved a meaningful and reproducible reduction of the kaolin viscosity, and have failed to significantly increase the brightness of the kaolin. Thus, there remains a need in the art of kaolin beneficiation for a process which can successfully remove mineral slimes from a kaolin clay. Such a process would permit the utilization of marginal and submarginal quality kaolin clays which formerly have been of limited or no commercial value.